A Comparison of Plume Rise Predictions in a Modelled and Observed Nocturnal Boundary Layer

Plume rise is a key variable in air quality modelling. The plume rise model implemented in CSIRO’s The Air Pollution Model (TAPM) was implemented in MATLAB, and the plume rise through modelled and observed boundary layers compared for an overnight strongly stable period. Contrary to expectations, the plume rise height was similar in the modelled and observed boundary layers despite a large difference between the wind and temperature profiles in the two cases. In the more weakly stratified modelled boundary layer, the plume was bent over by the stronger winds and there was a much greater rate of entrainment of ambient air into the plume. This enhanced mixing limited the height of rise even though the ambient stratification was much weaker. In contrast, with the observed profiles the plume rose almost vertically, because the winds were light in the deeper nocturnal inversion, and there was a much smaller entrainment flux into the plume. Even though the plume rise height was similar in the two cases, the volume flume in the plume was much less for the observed boundary layer structure so the plume was less diluted. Based on the results of this case study, it is suggested that assessments of predicted air quality impacts need to take into account the probability that models may not adequately predict the atmospheric boundary layer structure under strongly stable conditions.